Ethanol is a major chemical used in human beverages and food, as an industrial chemical, and as a fuel or a component in fuels, such as reformulated gasoline to reduce emissions from automobiles. This invention relates mainly to the production of ethanol for use as a chemical or fuel.
There are several traditional ethanol processes based on fermentation of carbohydrates. In the most typical process, a carbohydrate derived from grain is hydrolyzed to its component sugars and fermented by yeast to produce ethanol. Carbon dioxide is generated in the process from a fraction of the carbohydrate by the metabolism of the yeast. The generation of carbon dioxide is inherent in the metabolism of the yeast. This production of CO2 by yeast limits the yield of ethanol from yeast to about 52% maximum on a weight basis. This is a major limitation on the economic production of ethanol as the CO2 is of low value and is typically wasted into the atmosphere and may become a burden on the environment.
In addition, yeast have a limited ability to utilize sugars other than glucose. While glucose is the major sugar produced from the hydrolysis of the starch from grains, it is not the only sugar produced in carbohydrates generally. A large research effort has gone into the potential conversion of biomass into ethanol. Biomass in the form of wastes from agriculture such as corn stover, rice straw, manure, etc., and biomass crops such as switch grass or poplar trees, and even municipal wastes such as newspaper can all be converted into ethanol. However a major limitation of these processes is the complexity of the hydrolyzate that results from treatment of the biomass to produce the fermentation medium. The hydrolyzate typically contains glucose, but also large amounts of other sugars such as xylose, which yeast cannot metabolize. This is another potential yield limitation on yeast based ethanol processes.
Research has been directed to the use of organisms other than yeast which in contrast to yeast, do consume many if not most of the sugars derived from the hydrolysis of biomass. Examples include Zymomonas sp. bacteria and E. coli bacteria which have been genetically engineered to utilize xylose. Thereby the potential range of substrate sugars which can be converted to ethanol has been increased. There is a class of organism that has been proposed for the production of ethanol, typically of the Clostridium sp. These thermophiles usually produce both acetic acid and ethanol. However, it is believed that these organisms produce a limited yield of ethanol. It is generally assumed in the literature on ethanol fermentation that this yield limitation is fixed by the biochemical pathway called the Embden-Myerhof pathway by which ethanol is produced in all of the organisms so far proposed for production of ethanol, including the thermophiles.
Thus none of this development has addressed the inherent problem of the yield of ethanol from sugar based on the coproduction by the organisms of CO2.
An important part of the commercial processes for producing ethanol is the production of valuable coproducts mainly for use in animal feed or food. In the corn dry milling process the coproducts include distillers dried grains and solubles (DDG, DDGS). In the corn wet milling process the coproducts include germ, gluten meal and fiber. These coproducts find large markets in the animal feed business. However in both processes to a very large extent, the ingredients in the original grain, that is the oil, protein and fiber fractions, are passed through the processes unchanged in composition, while the carbohydrate fraction is converted largely to ethanol. Therefore the value of these coproducts is based on the inherent composition of the plant components.
There are other chemicals that can be produced by industrial fermentation from carbohydrates besides ethanol. Major examples are acetic acid and lactic acid. Acetic acid is a major food ingredient in the form of vinegar and a major industrial chemical. Vinegar for food use is typically produced from potable ethanol by the action of Acetobacter sp. which oxidize ethanol to acetic acid using oxygen from the air.
Major industrial uses for acetic acid are as a solvent, as an intermediate in the synthesis of other chemicals such as vinyl acetate and in the production of cellulose acetate. Major new uses for acetic acid have been proposed such as the production of calcium magnesium acetate (CMA) for use as a road deicer in place of sodium chloride (NaCl). CMA has a much reduced environmental impact compared to NaCl since it is much less corrosive and is biodegradable.
Researchers have proposed the production of industrial grade acetic acid by fermentation from carbohydrates. However no production by fermentation currently exists due to economic factors related mainly to recovering acetic acid from dilute fermentation broths. Acetic acid is typically produced at low concentrations of around 5% or less in water as a fermentation broth. Since acetic acid has a higher boiling point than water, all of the water, about 95% of the broth, must be distilled away from the acetic acid to recover the acid or other more complex processes must be used to recover the acetic acid.
Related to this field of acetic acid production is the use of so called acetogens, a class of bacteria which utilize a unique biochemical pathway to produce acetic acid from sugars with 100% carbon yield. For example, one mole of glucose can be converted to three moles of acetic acid by Clostridium thermoaceticum. These bacteria internally convert CO2 into acetate. These bacteria are called homofermentative microorganisms or homoacetogens. They do not convert any of the carbohydrate to CO2 and only produce acetic acid. Examples of homoactogens are disclosed in Drake, H. L. (editor), Acetogenesis, Chapman & Hall, 1994, which is incorporated herein by reference in its entirety. In addition these homofermentative organisms typically convert a wide range of sugars into acetic acid, including glucose, xylose, fructose, lactose, and others. Thus they are particularly suited to the fermentation of complex hydrolyzates from biomass. However this line of research has not overcome the economic limitations of the acetic acid fermentation process to make it competitive with the natural gas based route.
Therefore, industrial acetic acid is today made from coal, petroleum or natural gas. The major process is the conversion of natural gas to methanol and the subsequent carbonylation of the methanol using carbon monoxide directly to acetic acid. U.S. Pat. No. 3,769,329 describes this process.
Related to the natural gas route, it has been proposed to produce ethanol from acetic acid by way of synthesis of esters of acetic acid produced in this process, or a related modification, and subsequent hydrogenation of the esters. U.S. Pat. Nos. 4,454,358 and 4,497,967 disclose processes to produce acetic acid from synthesis gas, which is then esterified and hydrogenated to produce ethanol, and are incorporated herein by reference in their entirety. The hydrogenation of esters to produce alcohols is well known. None of these processes are based on the conversion of carbohydrates to ethanol.
There is another class of well known fermentations that have the property of converting carbohydrates at 100% carbon yield, using homofermentative lactic bacteria. These bacteria convert one mole of glucose for example into two moles of lactic acid. The relevance of this is that lactic acid may also be used as the substrate for fermentation to acetic acid by homofermentative acetogens again with 100% carbon yield. Two moles of lactic acid are converted into three moles of acetic acid by Clostridium formicoaceticum for example. Prior to the present invention, no one has been known to have devise a process to produce ethanol in high yield from carbohydrates, which is the main objective of this invention.